refactor(hugr-py): common up a null check

[ss2165]

avoids some bugs I kept making because of name similarities

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.64%. Comparing base (2bb079f) to head (4ede52c).

Files	Patch %	Lines
hugr-py/src/hugr/_ops.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1205      +/-   ##
==========================================
+ Coverage   86.62%   86.64%   +0.01%     
==========================================
  Files          96       96              
  Lines       18253    18248       -5     
  Branches    16831    16831              
==========================================
- Hits        15812    15811       -1     
+ Misses       1667     1663       -4     
  Partials      774      774              

Flag	Coverage Δ
python	89.13% <90.00%> (+0.24%)	⬆️

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